As the electronic products become smaller and smaller, the available surfaces of printed circuit boards for mounting semiconductor packages become smaller and smaller as well. The conventional side-by-side placement of semiconductor packages on a printed circuit board can not meet the requirements for miniature electronic devices. Therefore, 3D stacking technologies of semiconductor packages are developed to vertically stack a plurality of semiconductor packages to become a Package-On-Package device (POP), to meet the requirements of smaller footprints with higher density of components. In order to consider the cost and the utilization of the existing equipment, leadframe-based packages can be stacked by soldering the external leads of leadframes.
As shown in FIG. 1 and FIG. 2, a conventional stacked assembly 100 of semiconductor packages primarily comprises a first semiconductor package 110 and at least a second semiconductor package 120 stacked on top of the first semiconductor package 110. The first semiconductor package 100 is a conventional leadframe-based package comprising a chip 111, a plurality of external leads 112 of a leadframe, and an encapsulant 113 where the external leads 112 are configured for surface-mounting on an external printed circuit board, not shown in the figure. The second semiconductor package 120 comprises a chip 121, a plurality of external leads 122 of a leadframe, and an encapsulant 123. The external leads 122 of the second semiconductor package 120 exposed from the encapsulant 123 are formed in “I” shape are electrically connected to the external leads 112 of the first semiconductor package 110 by soldering materials 130 at the soldered sections of the external leads 112 close to the encapsulant 113. However, since the soldering area is small and narrow, the leads 122 are easily shifted due to thermal or mechanical stress. When the conventional stacked assembly 100 encounters thermal cycles, thermal shock, or drop test, the soldering materials 130 or the soldered sections between the leads 122 and leads 112 are easily broken.